The invention relates to a displacement machine for compressible media, in particular a dry-running vacuum pump, having at least two shafts with rotors, which are configured as profiled bodies and whose profiles engage with one another in the manner of gearwheels during rotation and run without contact relative to one another, each of the shafts being driven by its own electric motor, the angular positions of the shafts being determined by synchro resolvers, on the basis of whose signals the motors are electronically synchronized, and the shafts having gearwheels, which engage with one another and whose angular clearance is smaller than that of the profiled bodies.
For a long time, it was usual to synchronize the rotors of such displacement machines by means of gearwheels, only one motor being normally provided. It was, however, necessary to lubricate the gearwheels so that it was only possible to avoid pollution of the pumped medium by means of a very high level of complication in the sealing of the gear relative to the rotors and the actual pump space. However, the corresponding seals wear out so that the pump had to be taken apart in more or less regular intervals in order to replace the seals.
These problems are avoided, in a pump of the type mentioned at the beginning (U.S. Pat. No. 5,836,746), by each of the rotors being driven by its own electric motor and these being electronically synchronized. The angular positions of the two shafts are continuously determined by synchro resolvers. The synchro resolver signals are supplied to an electronic unit, which drives the two electric motors synchronously in such a way that the rotors cannot come into contact. In order to ensure, under adverse operating conditions with the synchronization operating inaccurately, that the rotors do not come into contact, which would lead to damage to the surfaces of the same, a gearwheel is provided on each shaft in this pump. The two gearwheels engage with one another and have a smaller angular clearance than the profiled bodies. If, therefore, the electronic synchronization fails, the gearwheels, which run without contact in normal operation, come into contact first. The profiled bodies, however, still cannot come into contact because they have an angular clearance which is greater than that of the gearwheels.
One problem in a displacement machine of this type consists in setting the rotors and gearwheels in such a way that, during operation, the flanks of both rotors and gearwheels have the largest possible distance apart. In the ideal case, the position of the rotors and the gearwheels should be such that the rotors take up an angular position relative to one another which is located in the center between the two angular positions at which contact occurs. The same applies to the gearwheels. In normal operation, the displacement machine would then be operated with this xe2x80x9cnull positionxe2x80x9d. This setting, however, is very difficult to effect. A setting operation by mechanical means is laborious and inaccurate because the angular clearance between the gearwheels and between the rotors is only very small, and must only be very small, so that the gap between the profiled bodies, through which a reverse flow occurs during pumping, is as small as possible.
In a displacement machine of the type mentioned at the beginning (U.S. Pat. No. 5,417,551 A), setting of the gearwheels to the average of the angular positions at which the gearwheels come into contact does in fact occur. The citation does not, however, show how the critical setting can be achieved such that the average of the two angular settings at which the flanks of the rotor come into contact coincides with the average of the corresponding angular positions of the gearwheels. It is only then, however, that unproblematic operation is possible. The citation only describes how the average of the angular positions of the gearwheels is set. It is then assumed that the average values for the rotors and the average values for the gearwheels coincide. It is, indeed, stated that the clearance of the gearwheels on one side must not be larger than that on the other because otherwise the rotors would come into contact. Such a problem, however, only occurs if the central points of the rotors and the gearwheels do not coincide. If this problem occurs, this can only take place by a relative angular adjustment between the rotors and the gearwheels but no information on such an adjustment is provided by the citation. In addition, such an adjustment may not be possible because the gearwheels are located far within the machine, the angular sensors are located more or less at the end of the corresponding shaft and the gearwheel and rotor of the corresponding angular sensor are not directly connected to one another.
The object of the invention consists in creating a displacement machine, of the type mentioned at the beginning, in which the xe2x80x9cnull positionxe2x80x9d of the rotors and the gearwheels can be set simply, rapidly and accurately by means of the synchro resolvers.
The solution according to the invention consists in the fact that at least one of the gearwheels is directly connected to the rotor of the synchro resolver of its shaft and both together are releasably connected as a unit to the shaft.
The setting of the xe2x80x9cnull positionxe2x80x9d and of the flank clearance takes place, in accordance with the invention, in the following way. One rotor, to which the gearwheel and the synchro resolver are releasably fastened, is first held steady. The gearwheel can then execute a rotary motion relative to the shaft of this one rotor. The other rotor is then rotated in both rotational directions as far as a position in which the flanks of the profiled bodies come into contact.
The two contact angles are measured, and the rotor with its gearwheel fastened to it is set to the central position between these two angles and held steady.
The first rotor, likewise, is still held steady. The gearwheel of the first rotor, however, is now rotated in both directions to the point where, in each case, it comes into contact with the gearwheel of the other rotor. The contact angles are likewise again established. The gearwheel is then set to the central value between these two contact points and firmly connected to the corresponding shaft; it is, in particular, firmly clamped by tightening bolts. Both the rotors, or profiled bodies, and the gearwheels are therefore located exactly in the central position between the two positions in which they are in contact or would be in contact. This is the null position used to carry out the synchronization, the control being carried out in such a way that the relative position of the two shafts, rotors and gearwheels corresponds, as far as possible, to this value during continuous operation.
The gearwheels are advantageously attached at one shaft end because they are then particularly easily accessible, which facilitates the fastening of the initially loosened gearwheel to its shaft.
The displacement machine advantageously has a differential control for the rotational speed of the motors. Well-synchronized operation has already been achieved by the synchronization according to the invention, be means of the setting of the flank clearance and the xe2x80x9cnull positionxe2x80x9d. The operational behavior is further improved if the two rotors are synchronized not to an independently specified required value but, rather, if the synchronization takes place mainly on the basis of differences in the angular positions. If, for example, liquid penetrates into the pump space, the rotors are greatly retarded because the density of the liquid is approximately a thousand times greater relative to gases with the retardation, however, taking place approximately equally for both rotors. Compensation for possibly occurring differences can then be provided by means of the synchronization. This would not be the case if the synchronization were to take place to an externally specified value. However, an additional external control does, of course, occur in order to permit input of the desired rotational speed. This control, however, which acts in the same sense on both motors, is relatively slow so that rapid rotational speed differences are obviated by the differential control.
It has been found particularly expedient for the drive if the motors are three-phase motors with permanent magnet rotors.